The present invention relates to an electronic function module for generating a current with a predetermined relationship to another current.
More particularly, the invention seeks to provide a function module for implementing the relationship:
y=ck/j
in which x represents a first current, y represents a second current and k and j are two integers whose ratio defines the exponent of the value x. As a result the function module according to the invention will be able on the basis of a first current x, to generate another current y, which can be any rational power of the first current.
A function module of this type has been described in an article by X. Arreguit, E. A. Vittoz and M. Merz, published in IEEE Journal of Solid State Circuits, Vol. SC-22, No. 3, June 1987, especially intended for incorporation in a data compressor applied to a hearing aid.
FIG. 4 of this article shows an embodiment of such a function module in which compatible bipolar transistors (or compatible planar bipolar transistors in CMOS technology) are used to establish the relationship between the two currents. The exponent of the value of the first current is determined by a resistive component and it is proposed to vary its value to enable a variable exponent value to be obtained. More particularly, a bank of resistors in series is provided and the resistors can be selectively connected in circuit with the aid of MOS selection transistors.
The known function module suffers from the disadvantage of requiring not only compatible bipolar transistors but also resistance components, which are poorly compatible with recent technologies for making exclusively CMOS circuits devoid of any resistance components. Moreover, applications of such a function module are limited, on the one hand because the value of the variable exponent has to lie between 0 and 1 and on the other band because of the various precautions which have to be taken to observe the characteristics of compatible bipolar transistors. The object of the invention is to provide a function module of the type briefly specified above but which does not have the disadvantages of the prior art. In particular the function module according the invention is perfectly adapted to modem technologies for implementing CMOS circuits and does not comprise any components other than MOS transistors.
The invention thus provides an electronic function module comprising a series of sections (C1, C2, . . . , Cj, . . . ) enabling the generation of a second current having a relationship to at least one first, current of the type y=xi, where x represents the value of the first current, y represents the value of the second current and i the order of the section in said series, characterized in that each section Cj comprises:
a pseudo-conductance G*j connected between a supply voltage (V*in) and a pseudon-grounded (7) and generating an output current (Ij);
a control transistor (Tj) passing the output current Ijxe2x88x921 of the preceding section Cjxe2x88x921 and adapted to control said pseudo-conductance G*j in such a manner that said output current Ij is proportional to the current Ijxe2x88x921 of the preceding section Cjxe2x88x921; and
a current conveyor (T3, T5, T6) for passing said output current Ij on the one hand to said control transistor of the following section Cj+1 and on the other hand to an output of the section Cj;
and in that the current passing through the control transistor of the first section C1 of said series is a fixed current (I0), such that the output current Ij of any section Cj of the series is proportional to I0j.
The invention also provides a function module comprising a series of sections whose characteristics are as specified above and enabling the generation of a second current which has a relationship to a first current of the type y=xk/j, where x represents the value of the first current, y represents the value of the second current and k and j are the orders of the sections Ck and Cj respectively, characterized in that it further comprises a close loop control circuit (Tl) supplying said supply voltage (V*in) on the basis of an arbitrarily selected input current (Iin) and the output current (Ij) of any section Cj of said series, such that the currents Iin and Il are kept equal, so that the output current Tk of a section Ck is such that Ik=Iink/j.
By virtue of these characteristics it will be possible to draw a current y from a given section in said network which will be a given rational power of the current output in another section, the power being determined by the ratio between the orders occupied by these sections in the network.
The function module according to the invention thus provides a large choice of current values having the desired power relationship between them, easily obtained by simply making connections.
Furthermore, it is found that the function module can be realized entirely according to CMOS technology, without the need for any resistive components.
The function module according to the invention can also have one or more of the following characteristics:
the closed-loop control circuit is formed by a single MOS transistor which provides a supply voltage for the pseudo-conductances with a value such as to ensure equality between an output current of a selected section and a given input current;
the pseudo-conductances are each formed by an MOS transistor biased so as to operate in a state of weak inversion;
the current conveyors are formed with the aid of current mirrors with two outputs.